*
 * Each instance processes a single event on [call], and then adds a successor [EventListenerRelay]
 * on the same [call] to process the next event.
 *
 * By forcing the list of listeners to change after every event, we can detect if buggy code caches
 * a stale [EventListener] in a field or local variable.
 */
class EventListenerRelay(
  val call: Call,
  val eventRecorder: EventRecorder,
) {

    when (listenerInstalledOn) {
      ListenerInstalledOn.Call -> {
        addEventListener(eventRecorder.eventListener)
      }

      ListenerInstalledOn.Relay -> {
        addEventListener(EventListenerRelay(this, eventRecorder).eventListener)
      }

      ListenerInstalledOn.Client -> {} // listener is added elsewhere.
    }
  }

  enum class ListenerInstalledOn {
    Client,
    Call,